A switching circuit, such as a relay or a triac, is typically employed to switch high voltage/power circuits with a lower voltage/power control signal. The control signal is generated by a secondary (control) device. Current switching applications (for example a Class 2 application switching a voltage less than 30V) typically use switching technologies including relays or triac devices. Other applications may include opto-isolated Field Effect Transistors (FET); typically, these circuits are limited to maximum load currents of a few milliamps (mA).
FIG. 1 is a schematic representation of an exemplary relay switch 100 that works through energizing (VCon) a coil 110 that acts as a magnet to pull down a gate 120 that connects a high voltage (VHigh) to the power circuit and enables a current flow. Latching relays (not illustrated) can have one or two coils. An impulse closes the circuit and a feedback loop keeps the gate closed. A reverse pulse opens the circuit or a second coil is energized to open the circuit.
The following limitations with relays are based on the analysis of a Class 2 application operating below 30V alternating current (AC). These limitations may also apply to circuits operating outside the 30V AC range:                When used in an application such as thermostat control, the operating voltage is typically 24V and the dissipation is 140 mW (for a non-latching relay). The operating range of a thermostat is between 18 and 30V, and at 30V the power dissipation increases to 220 mW. The thermostat control can typically run three devices (i.e. three loads each having an associated relay) resulting in a total power dissipation of approximately 600 mW. This adds significant heat to a temperature sensitive thermostatic control.        A further limitation of relays is arcing. Arcing occurs when the load current momentarily bridges the air gap as the relay gate opens. This causes electromagnetic (EM) noise and radio frequency (RF) interference that can adversely affect the operation of the thermostat, or other devices, particularly RF devices. In addition, when opening the relay gate, the sudden cutting off of control current in the relay coil also causes a momentary voltage spike in the control circuit potentially causing failure in the electrical components of the device.        Secondary parts such as voltage suppressors can be used to reduce the voltage arcing, although these add to cost and space requirements on circuit boards.        A relay can also degrade over time and may be ineffective when switched from a high power to a low power application. The contact surfaces wear out which degrades their ability to form a proper contact in a low power application.        The relay is also limited in the number of times it can switch in a lifetime, typically from 100K to 1M operations.        Switching of the relay is limited to a few cycles per minute.        In the event of a controller failure, the coil may be latched and continue running the appliance indefinitely (applies to latching relays only).        There is no inherent short circuit protection on a relay device.        Relays (regular and especially the latching type) are typically more expensive and occupy more volume than corresponding solid state devices.        
The following limitations with triacs are based on the analysis of a Class 2 application operating below 30V AC. These limitations may also apply to circuits operating outside the 30V AC range:                Triacs can only operate in an AC application (i.e. with an AC powered load).        Triacs require a switching current and have a typical voltage drop of 1-2V. They are not suitable for millivolt (mV) applications.        A limitation to triacs also relates to brownout conditions. In a brownout condition, the controlled voltage can drop to 18V. If a triac operates with a 2V drop, an overall 16V signal may be too low for proper operation.        Since the control signal is 5 to 20 mA, the heat dissipation can be significant.        Triacs usually require secondary circuitry to isolate the source and switching voltage. This is commonly done with opto-couplers which add to overall costs of the device.        By way of an example, a Triac switching 300 mA of current per circuit with 3 circuits active at once having a 2V drop will dissipate 1.8 W of power, which will add significant thermal offset to a thermostat application where accurate temperature readings are desired. In comparison, an exemplary MOSFET circuit in a similar application will dissipate 0.054 W of power.        Triacs may have leakage current through the device. In a low power application, the small (leakage) current may be interpreted as a false signal.        
What is needed is a switching mechanism for switching high voltage/power circuits with a lower voltage/power control signal that mitigates some or all of the disadvantages described above.